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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Numerical Analysis of Foundation Slabs
T. Krejcí, T. Koudelka, J. Šejnoha and P. Kuklík
Department of Structural Mechanics, Faculty of Civil Engineering, Czech Technical University in Prague, Czech Republic
T. Krejcí, T. Koudelka, J. ÂŠejnoha, P. Kuklík, "Numerical Analysis of Foundation Slabs", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 63, 2006. doi:10.4203/ccp.83.63
Keywords: heat and moisture transport, hydration heat, drying and shrinkage of concrete, damage.
In this paper, we present the computer simulation of concrete slab behavior in an early stage. The concrete slab has a thickness of 1m and it is reinforced in both directions. It was cast in three layers. The slab was covered with a plastic foil for the next five days to prevent quick evaporation of the redundant water. The computer simulation starts one day after the concrete slab was finished, until we assume the end of concrete setting process.
The foundation slab is situated in the basement of the administration building Tešnov in Prague. The Künzel and Kiessl's model  was used to analyze heat and moisture fields and was extended by a set of climatic conditions and by a model of hydration heat evolution in concrete.
It is evident from the relative humidity profile that the evaporation process starts on the upper surface after removing the plastic foil. We obtained a very good correspondence in temperature from the computer simulation and in situ measurements.
Shrinkage and creep of concrete together with free thermal strains have a significant influence on the total strain, which is decisive for creation of the crack. It is possible to use B3 model for simulating shrinkage and creep of fresh concrete. It is described for example in [2,3]. Both effects are included in this model and they are temperature and moisture dependent.
A series of material models were developed for simulating of damage of concrete. The simplest one is a scalar isotropic damage model. The accuracy of this model is highly dependent on the equivalent strain norm, on which the damage parameter depends. The Mazars norm was selected as convenient norm for concrete.
We can conclude two things from the results of the analysis. The first is that the accumulated hydration heat expires approximately after seven days, and the autogeneous shrinkage phase ends, too. The second conclusion is that during the process of drying, the drop in moisture content and temperature occurs first in the surface layers and only much later in the core. The effect of the diffusion process of drying (shrinkage of concrete) on the stress development and micro-cracks distribution is rather extensive. Smeared cracks can cause the on-set of main cracks.
This study shows that the modeling of deformational and drying processes in new concrete demands the choice of a physically correct model describing the transport processes, the choice of real material parameters for heat and moisture transport (including model of heat hydration evolution), the choice of a suitable damage model for concrete, and a set of correct initial and boundary conditions (climatic conditions).
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